1. Field of the Invention
The present invention relates to a hologram reconstructing apparatus and method that reconstruct data multiplexed on a hologram recording medium by angle multiplexing and in particular to ensuring the compatibility when a hologram recording medium is reset or is replaced by a hologram recording medium recorded by a different apparatus.
2. Description of the Related Art
From the past, in a recording operation on a hologram data storage, a hologram recording medium is placed in an area where two ray bundles of signal light and reference light interfere, and the interference fringes of the two ray bundles are recorded on the hologram recording medium. In this case, in order to improve the recording density on a hologram recording medium, hologram multiplexing is performed by various kinds of multiplexing such as angle multiplexing, shift multiplexing, speckle multiplexing and shift-phase code multiplexing.
In a hologram recording apparatus by angle multiplexing, two ray bundles of signal light and reference light are irradiated to a hologram recording medium. Every time the signal light is modulated spatially (modulated in intensity) on a data page displayed by a spatial light modulator (SLM), the incident angle (recording angle) of the reference light to the hologram recording medium is changed, and different pieces of record information are multiplexed on one same recording area.
In a hologram recording apparatus by angle multiplexing, multiple, such as 40, pieces of different record information can be recorded in one same recording area by changing the change in pitch of the recording angle of the reference light, such as in increments of 0.1 degree in the range of several degrees at maximum (by changing slightly in the range of 88 to 92 degrees, for example) and sequentially multiplexing different pieces of record information (data pages) in one same recording area. A set of data pages recorded in one same place is called “book” as required. One resulting from recording only one book is called “single book”, and one resulting from recording multiple books is called “multibook”.
On the other hand, a hologram reconstructing apparatus pairing with such a hologram recording apparatus is configured to reconstruct record information multiplexed in one same recording area by slightly changing the angle of the surface of a hologram recording medium against reference light for reconstruction. Here, the angle of the reference light for reconstruction against the surface of a hologram recording medium is called “reconstruction angle” as required in reconstruction with a hologram recording medium on which holograms are recorded by angle multiplexing. Furthermore, the reference angle of the reconstruction angle, such as the reconstruction angle equal to the normal of the surface of a hologram recording medium, for example, is called “reference reconstruction angle”. The hologram reconstructing apparatus by angle multiplexing can reconstruct the record information recorded by a recording angle equal to a reconstruction angle for each reconstruction angle by slightly changing the reconstruction angle from the reference reconstruction angle according to the recording angle.
With the hologram recording apparatus and hologram reconstructing apparatus by angle multiplexing, multiple pieces of record information can be recorded for each recording angle of reference light in one same recording area, and the multiplexed record information pieces can be reconstructed separately. Therefore, the recording density and recording capacity can be dramatically increased.
By the way, even small amount of displacement of the set position of a hologram recording medium between recording and reconstructing makes finding a recording surface difficult since a hologram is generally recorded three-dimensionally in a recording area of a hologram recording medium and the surface of the hologram recording medium is a two-dimensional recording area (recording surface). This is because the wave information of the reference light in recording and the reference light in reconstructing may change when the position of a hologram recording medium changes, which prevents obtaining a reconstructed image. Furthermore, in order to increase the recording density to a hologram recording medium, the space between recording areas is necessarily decreased. However, when the distance between recording areas decreases, the record information in the adjacent recording place may be possibly reconstructed or the record information by a different recording angle may be possibly reconstructed even by the slight displacement of a hologram recording medium from a predetermined position in reconstructing. Therefore, a hologram recording medium is needed to set at the same position as that in recording in order to reconstruct data on the hologram recording medium, and precise positioning is demanded. Otherwise, the compatibility may not be obtained when a hologram recording medium is reset or played by a different apparatus or when a hologram recording medium recorded by a different apparatus is played, which may be one factor responsible for the prevention of the practical use of the method.
Accordingly, as a reference recording angle, the recording angle is defined for recording data on a specific recording surface on which data is to be recorded by a first recording angle, for example, in a hologram recording medium. Then, the recording angles after data is recorded on the specific recording surface are changed and fixed by a predetermined angle with reference to the defined reference recording angle. Therefore, a publicly known example (refer to JP-A-2003-337524 (Patent Document 1)) can improve the compatibility by allowing the reference recording angle determined by the mechanical states or defined conditions of recording angle changing means and optical system, for example, in the hologram recording apparatus side to agree with the reference recording angle in the hologram recording medium side at a specific recording time such as the first recording time. However, in the publicly known example, the positioning method is complicated and is allowed in angles/directions only. Therefore, the technology is not applicable to the positioning control over a hologram recording medium.
For this reason, a method has been proposed (refer to JP-A-2006-171589 (Patent Document 2)) that can secure and stabilize the compatibility upon reset of a hologram recording medium or upon playback in a different apparatus by performing highly precise positioning between reconstruction reference light and the hologram recording medium, without complicating an apparatus to be used.